In the prior art, there is known, for example from the patent publications FI101200 B, FI 101199 B, FI 112039B, FI 112328 B and FI 113244 B, various methods and arrangements for separating two solutions mixed in dispersion into two solution phases in a liquid-liquid extraction separation cell. A first solution and a second solution, which is heavier than the first solution, can be separated from the dispersion of said solutions. Generally the cell arrangement includes side walls and a bottom, inside which there is defined a separation space. The cell has a feed end, through which the dispersion is fed into the cell, and distributed, by some suitable arrangement, evenly along the whole width of the cell. At the cell drain end, the first and second solutions are arranged to be removed as mutually separated. In between the feed and drain ends, the cell is provided with shut-off elements, by which the flow of the separating solution phases and dispersion is controlled; in between said shut-off elements, there are created successive separation steps, where the lighter first solution (generally an organic phase) is separated as an upper solution phase, and the second solution is separated below the upper solution phase as a lower solution phase (generally an aqueous solution). The cell drain end is provided with an overflow chute, which is positioned transversally with respect to the flowing direction, and receives the first solution separated into the upper phase as overflow from the cell, and the solution is drained from said overflow chute. In the flowing direction, in succession to the overflow chute and adjacently with it, there is provided a collecting chute for receiving the second solution as underflow from the cell. Riser pipes extend from the collecting chute to the cell, and through said riser pipes, the second solution can rise to the collecting chute, from which the second solution phase is drained.
A perpetual aim in liquid-liquid extraction separation is both to improve the feed-through capacity and to reduce mixing values, which aims have in practice been fairly contradictory. The term ‘mixing value’ here means the quantity of residual droplets from the other solution in the separated solution phase. When the capacity is increased, the mixing values tend to rise, because the liquid-liquid contact time is cut shorter. In the prior art, with a specific flow power of 4 . . . 6 m3/m2h, the object has been to achieve mixing values where the quantity of residual droplets from the organic matter in the aqueous solution phase (so-called “O/A entrainment”) is within the range 5 . . . 10 ppm, and the quantity of residual droplets from water in the organic phase (so-called “A/O entrainment”) is within the range 50 . . . 100 ppm.